It is known that a metal strip can be continuously treated with an acid or alkaline electrolytic treating liquid while moving the metal strip along a horizontal or vertical path provided between a pair of horizontal or vertical electrodes, either soluble or insoluble in the electrolytic treating liquid, by passing the electrolytic treating liquid through the gaps between each electrode and the metal strip and by applying a voltage between each electrode and the metal strip so as to generate a desired intensity of electric current therebetween. The electrodes may be either anodes or cathodes, whereby the metal strip serves as either the cathode or anode, respectively.
For example, in the case where a metal strip is continuously electroplated with zinc by using a horizontal-type electroplating cell, a pair of insoluble electrodes, and an acid electrolytic solution containing zinc, the amount of the electrodeposited zinc layer on the metal strip is governed by Faraday's law. That is, one faraday (96,500 coulombs) of electricity applied to the electroplating system results in deposition of one gram equivalent of the metal, that is, 32.5 g of zinc, on the metal strip. This electrodeposition phenomenon is governed by the following equation: ##EQU1## wherein, I denotes the intensity of electric current in amperes; W represents the width of the metal strip to be plated in mm; V represents the moving speed of the metal strip in m/min; Cw represents the weight of the electrodeposited metal layer in g/m.sup.2 ; and .eta. represents the current efficiency.
The value of W is determined by the width of the metal strip to be plated. The value of Cw is determined by the weight or thickness of the plated metal film instructed by the customer. The value of .eta. is determined by the type of metal to be electrodeposited.
Therefore, it is obvious that if one wishes to increase the productivity of the electroplated metal strip by increasing the moving speed of the metal strip, it is necessary to increase the value (I) of the electric current, to be applied to the electroplating system, in proportion to the increased moving speed of the metal strip.
However, it is known that when the electroplating procedure is carried out at a high current density, for example, 100 A/dm.sup.2 or more, use of a conventional electroplating apparatus suitable for a relatively low current density, for example, less than 100 A/dm.sup.2, results in undesirable so-called burnt deposites on the metal film electrodeposited on the metal strip. Also, for high current density to be used with conventional electroplating apparatus, it is necessary to apply undesirably increased voltage between the electrodes (anode) and the metal strip (cathode).
If it is desired to operate the continuous electrolytic treatment process at a high speed of 150 to 300 m/min using conventional electrolytic treating cells suitable for a relatively low current density of below 100 A/dm.sup.2, the only way to avoid the above-mentioned disadvantages would be to use a plurality of the conventional electrolytic treating cells. This would, however, result in high costs.